1. Field of the Invention
This invention relates to multi-stage gas-fired furnaces and, more particularly, to a method for proving the high-heat pressure switch in a multi-stage gas-fired furnace.
2. Description of the Prior Art
During certain situations in the operation of a multi-speed furnace, problems can arise in that the high-fire pressure switch (HPS) may fail to close or may, due to an increase in pressure in its immediate vicinity, open while in high-fire. The former situation can occur in longer length vent systems at high altitude and on furnaces with higher rise ranges. The latter situation can occur when the furnace is already operating in high-fire mode as previously requested, and may be caused, for example, by a high wind gust impinging a horizontal vent. In either case, when nothing is done about the HPS being open, the system will normally attempt to satisfy the thermostatically communicated high-heat demands using low heat if the HPS does not close. If successful, the furnace would be required to run for an excessive period of time in low-fire mode in order to satisfy the thermostat, and it will take longer than desirable for the temperature to reach the pre-set comfort level. When the furnace is recovering from night set-back the loss of high-fire heat may result in the system taking many hours to regain the desired temperature. In some instances, heat delivered in the low-fire mode may not be sufficient to satisfy the thermostat and the temperature in the conditioned space will become low enough to cause occupant discomfort.
In the prior art, in particular copending U.S. patent application Ser. No. 08/090,332, assigned to a common assignee, an interlock is provided between the high-fire pressure switch and the high-fire solenoid to prevent the high-fire solenoid from being energized when the furnace is in low fire mode. Twenty four volt thermostat power is denied to the high-fire pressure switch and high-fire solenoid whenever there is a call for low heat.
In U.S. Pat. Nos. 4,982,721, 5,027,789 and 5,186,386 all to Lynch, the system attempts to deal with the problem of the high-fire pressure switch remaining closed (causing the system to run in high-fire mode with respect to the amount of fuel delivered) when the inducer fan is running at low speed--that is the combustion air is being delivered at a volume appropriate for low-fire mode. This is done before gas ignition is attempted by running the inducer fan on low speed for 1 minute, turning off the inducer fan for 4 minutes, and running the inducer fan on high speed for 15 seconds before starting another cycle.
None of these documents address the problem of the high-fire pressure switch failing to close when it should, or reopening during high-fire mode operation.